A hemolytic reaction occurs when the body’s immune system mistakenly attacks and destroys red blood cells. This can happen during a blood transfusion if the donor's blood type is incompatible with the recipient's. The immune system identifies the foreign blood cells as threats, leading to their rapid breakdown, which can cause symptoms like fever, chills, and dark urine. In severe cases, a hemolytic reaction can lead to serious complications, such as kidney failure or shock. It's crucial for medical professionals to carefully match blood types, such as A, B, AB, or O, to prevent these dangerous reactions during procedures like blood transfusions.